Crews work to try to prevent a dam from failing at Lake Tangipahoa in Mississippi's Percy Quinn State Park Aug. 30, 2012. (WAFB-TV)

NEW ORLEANS - Officials on Thursday began a controlled release of water at a lake dam that threatened to break near the Louisiana-Mississippi border flooding a rural area where thousands of residents live.

Louisiana Gov. Bobby Jindal called the situation in the Tangipahoa Parish area "the biggest challenge" the state faced Thursday and said 40,000 to 60,000 would be affected if the dam breaks. Search-and-rescue teams have been evacuating residents within a one-mile area throughout the day. Two nursing homes also have been evacuated.

Jindal said that if the water hadn't been released it would have caused significant flooding -- with water pouring into the already swollen Tangipahoa River, swamping low-lying areas downstream. Residents had less than 90 minutes to evacuate after the order was given, Jindal said at a briefing Thursday.

Jindal said he went on a helicopter fly over of the river. "You could see the water spilling over," he said.

Experts for the U.S. Army Corps of Engineers examined the dam, FEMA Deputy Director Richard Serino said.

If the dam were to burst, Jindal said, residents could see floodwaters as high as 17 feet - similar to floods in 1983 and 1990.

Amite Police Chief Jerry Trabona said officers were going from house to house along both sides of the river, notifying residents of the river threat.

"If they don't have a way out, we will bring them out of there," Trabona said.

Meanwhile, the National Guard was evacuating 3,000 people trapped by flooding in LaPlace, La., the governor's office said. Jindal said they were rescuing about 30 people an hour.

Storm surge from Lake Pontchartrain and Lake Maurepas flooded St. John the Baptist Parish.

Rising water closed off all main thoroughfares into the parish, which is about 30 miles west of New Orleans. In many areas, water lapped up against houses and left cars stranded. The water was being driven higher by south winds as Isaac passed to the west and was expected to continue rising through the night.

Floodwaters rose waist-high in some neighborhoods, and the Guard was working with sheriff's deputies to rescue people stranded in their homes and surprised by the flooding.

The state dispatched 89 buses to drive residents, including elderly people in an assisted-living facility, to shelters in Alexandria and Shreveport. The state's Department of Wildlife and Fisheries had 30 boats on-site and 20 others en route to help in the rescues.

The floodwaters "were shockingly fast-rising, from what I understand from talking to people. It caught everybody by surprise," Lt. Gov. Jay Dardenne said.

Isaac's battering winds finally had begun to soften a bit Thursday as the weather system slowly marched north, but rescue crews stayed busy across the region.

The hurricane-protection system ringing the New Orleans area continued to hold, keeping storm surge and floodwaters out of the city. But in its suburbs of LaPlace and Slidell, rescue crews were helping residents evacuate from flooded homes.

A Coast Guard helicopter crew rescued a couple and their two dogs from a flooded house early Thursday morning in LaPlace, Coast Guard officials said in a press statement.

A crew from the Coast Guard Air Station in New Orleans hoisted the husband and wife into an MH-65C Dolphin helicopter after receiving a report around midnight that the couple had been stranded.

"The husband and wife and their two dogs were in an area where a lot of houses washed away," said Lt. Cmdr. Jorge Porto, Air Station New Orleans pilot. "They used a flashlight inside the house as a signaling device, which made all the difference in locating them effectively."

Across Lake Pontchartrain in St. Tammany Parish, residents were also fleeing flooded neighborhoods.

Homes along Interstate 55 near New Orleans were steeped in floodwaters up to the roof awnings. One sheriff's deputy's car was stranded in floodwater near the highway.

Isaac hasn't completely loosened its grip on the region yet: Area rivers, steadily swelling with Isaac's rains, weren't expected to crest until the weekend, potentially flooding more homes and making more roads impassable, said Capt. Doug Cain, Louisiana State Police spokesman.

"We still have some challenges ahead of us," he said.

Statewide, 6,191 residents - with the number expected to grow - were in shelters, Jindal said.

Louisiana State Police stayed busy clearing roadways and making streets safer. State troopers are also tasked with helping local police secure homes and businesses, he said. So far, only a few isolated cases of looting have been reported.

"In the big picture, it's been minimal," Cain said. "We've been very happy with that."

Nearly half of Louisiana remained without power. The Public Service Commission says 901,000 homes and businesses around the state - about 47% of all customers - were dark.

In neighboring Mississippi, utility companies say they are working to restore power to more than 150,000 customers in the south and central parts of the state.

Airline, rail and automotive traffic was expected to remain snarled across several states through week's end.

President Obama expanded a declaration of federal emergencies in Louisiana and Mississippi late Wednesday, according to a statement from the White House. The disaster declarations free up federal aid for affected areas.

Parts of New Orleans have already been pounded with up to 17 inches of rain.

New Orleans Mayor Mitch Landrieu ordered a dusk-to-dawn curfew. "This is not the time to let your guard down. We're still in this thing, so it's more important than ever for residents to stay vigilant and remain calm."

State and parish officials on Thursday were studying the levees in Plaquemines Parish to figure out the best place to punch a hole in them to relieve trapped floodwaters that overran the enclave of Braithwaite.

Rescue crews evacuated Braithwaite residents in boats after a storm surge from Isaac overtopped levees there, trapping residents in homes and on rooftops.

Braithwaite's levees are smaller tidal levees, about 8 feet high, that ring the community and keep out water from high tide and smaller storms, said Bob Turner, regional director of the Southeast Louisiana Flood Protection Authority-East, who is assisting in the mission. They are not designed to keep out hurricane-strength surges, such as the 12- to 14-foot storm surge that barreled in late Tuesday, he said.

Once the floodwaters got in, the levees kept them in, Turner said.

"The system cannot get the water out by natural means," he said. "You either pump it out or punch a hole in the levees." He added: "There are no pumps large enough to do that in an efficient way."

Officials decided to breach the levee to free the water. In the coming days, an excavator will claw a trench into the levee most likely on the east side of Braithwaite, allowing the water to flow into a nearby diversion canal and sending it into the marshes of Black Bay and eventually out into the Gulf, Turner said.

No other homes should be impacted by the levee opening, he said.

Similar levee punctures were done after Hurricane Katrina in 2005 that allowed trapped floodwaters in New Orleans to escape, Turner said. The current operation is still a few days away, as weather and high water make it hard to reach the impacted area, he said.

"The idea is to get the water out as soon as possible so all your search and rescue is done and you can start the recovery process," Turner said.

Lingering effects

Flights to and from New Orleans were canceled for a second straight day Wednesday. Airports in other Gulf cities as far east as Pensacola, Fla., were closed. Southwest Airlines said all of its New Orleans flights were suspended at least through 5 p.m. Thursday. The vast number of cancellations - and the storm's trajectory - could cause travel delays across the nation heading into Labor Day weekend.

"We'll be dealing with this storm through early Friday morning," Louisiana Gov. Bobby Jindal said.

Most of Lakeshore Drive along New Orleans' Lake Pontchartrain is closed to traffic, but the storm surge remained below the levees. The Army Corps of Engineers' $14.45 billion overhaul of the area's hurricane protection system was holding back surges and floodwaters, said Bob Turner, regional director of the Southeast Louisiana Flood Protection Authority-East.

Throughout the New Orleans area, neighborhoods swallowed by Katrina's catastrophic floods were drenched by Isaac but spared major flooding. The Lake Borgne surge barrier, a $1 billion structure erected after Katrina, stopped a 15-foot storm surge headed to the Lower 9th Ward- perhaps the hardest hit area in 2005 - and other parts of the city.

Without that 26-foot-high barrier, stormwater would have topped levees and flooded neighborhoods ravaged by floods during Katrina, he said. "You would have had water flowing in the Lower 9th Ward again," Turner said. "The barrier did its job."

Lower 9th Ward resident Gloria Guy spent 9½ hours on the roof of her flooded home during Katrina before she was rescued. On Wednesday, Guy said she had mostly slept through Isaac in a home built after Katrina by actor Brad Pitt's Make It Right Foundation.

"Much better than Katrina," said Guy, 72. "Besides not having any lights, everything's fine."

Just on the other side of the gate from Braithwaite and inside the hurricane protection system, David Manes, 33, rode out the storm at home with his three young sons. Isaac's winds snapped trees in half and peeled back some of his roof's overhang. Isaac's muscle caught Manes off guard.

"It wasn't supposed to be this bad," he said. "If I had known it would've been this bad, I would've stayed with my mother in Mississippi."

New Orleans' historic French Quarter appeared to have dodged the worst of Isaac. Downed tree limbs, minor flooding at intersections and a brief electrical outage overnight were the main problems confronting the residents who stayed.

"Honestly, man, it's just been rain," said Huggington "Huggy" Behr, manager of Flanagan's Pub, which remained open through the night. "To us, we've seen the worst, so it's business as usual."

New Orleans businesses fretted that the lingering storm would hamper this weekend's three-day Southern Decadence gay celebration, which organizers say draws up to 100,000 visitors. Round-the-clock activities are scheduled, mostly around the dozen French Quarter bars and adjoining neighborhood.

Sharon Senner, owner of Chateau Hotel in the Quarter, said all 49 rooms were fully booked for the weekend, but half the guests canceled or inquired about canceling.

The Red Cross- already housing 5,000 evacuees in 80 statewide shelters - is preparing a prolonged recovery. "We're going to be there for weeks," Red Cross Vice President Charles Shimanski said. "We need to know what we're recovering from before we know what recovery looks like."

Residents weary of floods

Isaac will never compare to Katrina's ferocity, but its slow, wobbly march north is prolonging another round of agony for thousands along the Gulf Coast.

David and Gina Oliver were sitting on their front porch in Slidell, La., on Wednesday afternoon when they saw the water climbing up to their yard.

"Thirty minutes later, the house was full," Gina Oliver said.

The couple evacuated with their two kids to her mother's house farther inland. But they were back Thursday afternoon, loading their small fishing boat onto a flooded road to get back to their house and salvage anything they can.

"I don't wish bad on anyone, but they fixed New Orleans. They're high and dry," Gina Oliver said. "And still, all you hear is New Orleans, New Orleans, New Orleans. You haven't heard a damned thing about Slidell."

Richard Roussell, 45, a construction worker in Slidell, walked along the train tracks on Front Street after checking in on his house. He evacuated before the storm hit, but on Thursday morning he walked a mile on the tracks to check on the house he'd bought six months ago in a foreclosure. He had just finished renovating his new home two weeks ago.

"I thought it was a little baby storm, but obviously I was wrong," he said. His home had about a foot of water inside. "It ain't pretty," he said.

Measuring risk in Mississippi

Gov. Phil Bryant said the state had been relatively fortunate so far, but he worried that people were acting too casual as the weakened storm moved farther west. With Isaac's winds keeping the storm surge close to 10 feet throughout the coast, up to 3 inches of rain falling per hour and tornadoes spotted throughout the state, he urged people to stay inside.

"The surge continues. Unfortunately so does the rain and the wind," he said Wednesday afternoon from an emergency operations center in Gulfport. "People appear to be almost ignoring the tornado warnings. This is a very dangerous situation."

Seventy roads were closed near the coast and rescue crews on boats and National Guard trucks had rescued 58 people. Most of those rescues were in Hancock County, which borders Louisiana, where flooding was widespread.

Mississippi Emergency Management Agency (MEMA) Director Robert Latham said some houses that are flooded there were built on stilts after Hurricane Katrina.

"That shows you the significant height of the water," Latham said. "The storm surge may recede, but we've got a lot of rainfall still coming down that's going to keep those water levels pretty high."

"We really don't have a clue on damages yet," MEMA spokesman Jeff Rent said. "The storm is moving so slowly that it's going to be a while before we get out there to assess it. I can tell you've we've had a lot of roads in our coast counties that have closed due to debris and flooding."

Forecasters expected Isaac to move inland over the next several days, dumping rain on drought-stricken states across the nation's midsection before finally breaking up over the weekend.